Liang Bua, Flores, IndonesiaSoutheast Asia

The site of the Liang Bua cave in which Homo floresiensis inhabited if located on the island of Flores, Indonesia.  This region is a part of the Lesser Sunda islands in the eastern part of the Indonesian archipelago. The island is located in a region known as the Soa basin that features a mosaic of mountainous and volcanic environments with a variation of rainforest and grassland habitats (O'Sullivan, et al.) The island falls within the range of the region of Wallacea near the "Wallace Line" which is a biogeographical boundary that separates floral and faunal species from southeast Asia (Sunda) and Australia (Sahul) (O'Sullivan, et al.) 

In early hominin migrations out of Africa, the Wallace Line presented a challenge for early hominins to access such a habitat as well as to adapt to in terms of the environment and the limited resources on such an isolated island. Due to this low-species richness, many of the vertebrate fauna, including H. floresiensis show indications of being affected by an insular environment through occurrences of dwarfism and gigantism (Meijer et al. 2010).

In 2003, scientists found a new species in the Liang Bua cave on the Indonesian island of Flores. This new species became to be known as Homo floresiensis, a.k.a “the hobbit” because of their small size.  

            H. floresiensis was found to have lived ~100-50 KA. The most important skeleton found on the island was Liang Bua 1 (LB1) which gave us most of the information we have now. It is a partial skeleton but is comprised of a cranium, mandible, multiple lower limbs and was found in Sector VII about 5.9 meters deep in the excavation. In later excavations during 2004, a right humerus and incomplete right and left ulnae were discovered which were later found to be part of LB1. Other partial skeletons were found, most including upper portions of the skeleton, of individual people. They then found the evidence to be for LB2, LB3, LB4, LB5, and LB6.

H. floresiensis lived to be about one meter (3 feet) tall fully grown, had no chin, a long and low braincase, an extremely tiny brain size of about 440 ccs, and long feet compared to the femur and tibia. All pieces of skeleton found had a mix of primitive and derived features. Australopith-like (primitive) features include the long length of their arms, structure of the wrist, robusticity of limb bones, short stature, and small endocranial volume. More derived features include reduced facial prognathism, metatarsal robusticity sequence, facial height, fully adducted hallux, and small postcanine teeth. Because of the mix of derived and primitive features, their ancestry/origin has been hard to place.

When looking at the feet of H. floresiensis it has been concluded that they have descended from a primitive unknown species in Southeast Asia. More popular theories for their line of ancestry have postulated that they are descended from H. erectus after a long period of isolation. The original founders of the skeleton concluded that they were descended from H. habilis or a H. habilis-like hominid from East Asia. Using cladistic analysis, it was found that there were two most parsimonious options. Option 1 is that H. floresiensis had branched off after H. rudolfensis, before the branching of H. habilis. Option 2 is that H. floresiensis branched off after H. habilis. Both concluded that H. floresiensis was a new species and not a dwarfed modern human. Most importantly though, this cladistic analysis did not phylogenetically link H. floresiensis to H. erectus which is the most supported theory out today.

Since their discovery, there have been multitudes of theories suggesting the cause of their small stature. Some theories suggest H. floresiensis being closely related to modern humans however, with some form of genetic abnormality ranging from down syndrome, microcephaly, ME cretins due to hypothyroidism, or founders’ effect. While genetic explanations are the most popular, they are usually presented with little evidence and have no real basis. The theory of island insulated island dwarfism creating their diminutive size has proven to be the most likely. This theory proposes that they shrank because of the limited amount of resources available on the island and lack of predators. This theory holds strong as insulated island dwarfing is evident among other animals on the island including Stegodon florensis insularis (dwarfed elephant).

Flores has experienced rapid environmental and climate fluctuations over the last ~100ka due to a variety of factors. One of which is due to the fact that Flores lies along a highly tectonically active region with high rates of volcanism from the Banda volcanic arc that includes multiple volcanoes and volcanic islands 18. Due to this high tectonic activity, Flores is uniquely an isolated island due to "Wallace's Line" geographical barrier that has notably affected its accessibility as well as low variation of flora and fauna biodiversity. 19. Flores' position within the world's warmest sea water as from the Indo Pacific Warm Pool (IPWP) and the Intertropical Convergence Zone (ITCZ)  also makes it susceptible to major climatic and environmental fluctuations like monsoons and droughts 18.

 

The site of the Liang Bua cave have been subject to various climatic, geological, volcanic, and environmental shifts over the last ~200ka.  Speleothem oxygen isotopic evidence from cave stalagmites from the islands of Java and Flores have been used to proxy the significant paleoclimate changes that occurred on Flores during the occupation of Homo floresiensis at the cave site of Liang Bua from about ~100-50ka 17.

 

Initial isotopic analysis infers that during the previously thought period of H. floresiensis from ~49-13 ka, there was a very warm, wet phase that created organically rich soil conditions suitable for a wide range of vegetation in a closed woodland environment from ~49-36 ka 18. Following was a phase of decreased rainfall from  ~39-17ka. At this time the data indicates there may have been a decrease in water availability causing a gradual transition to more C4 types of vegetation that require less water and an overall decreased range of vegetation 18.  In the latter part of this dry period the environmental conditions would have shifted towards very dry and organically poor soil that may have led to a decrease in woodland type environments and an increase in grassland and even desert habitats 23; 24 25; 26; 27; 28.   This paleoclimate reconstruction suggests that the environmental conditions went from closed, woodland habitats during the earlier phases (~49-39ka) to quickly shifting to open and less forested ones between ~36-17ka.

 

Recent analysis reveals that faunal assemblages of H. floresiensis, Stegodon florensis insularis, along with giant marabou stork (Leptoptilos robustus), vulture (Trigonoceps sp.), and murine rodents may reflect accurate representations of the paleoecology of the site 16.  Data from these studies show the faunal structure of Liang Bua and surrounding parts of flores during the time of H. floresiensis' occupation as best adapted to open savannah grassland habitats 15 16.  

 

At ~60 ka, the environment  dramatically changed towards a closed woodland type habitat with organically rich soil and a higher variety of woodland type tree vegetation (Veatch et al., 2019).   This inference is based on paleoclimatic records of speleothem oxygen isotope data from the nearby Liang Luar cave and the Liang Bua cave that indicates a decrease in vegetation of C4 grasses to more C3 dominated forest habitats  in region at ~69-62 ka 20, 21, 22.  Prior and after this time period, there may have been a dominance of closed habitats near Liang Bua cave prior to H. floresiensis occupation as indicated by the higher frequency of forest adapted murines and other species at those times 20, 21, 22.

 

This ecological change is also indicated from the reduction of medium sized murines and the disappearance of large mammals like the Stegodon florensis insularis and H. floresiensis, which are assumed to have migrated elsewhere on the island in search of more open habitats they are adapted to inhabiting 16.   Other causes for the disappearance of fauna and H. floresiensis  at this site include the high amounts of volcanic activity as per the high levels of volcanic ash found near the sight, rapid climate changes, and even modern humans arriving on the island  during the time period of ~50-47 ka 17.  


It seems acceptable to assume that the high volcanic activity and drastic climatic changes occurring at ~60ka may have contributed to the possible disappearance of H. floresiensis and other faunal species occupying the site of Liang Bua cave.

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